High-performance liquid chromatographic separation of iodoamino acids for tracer turnover studies of thyroid hormones in vivo

Effects of tetraiodothyronine and triiodothyronine on hamster cheek pouch microcirculation

The aim of the present study was to assess the effects of topically applied triiodothyronine (T(3)) and thyroxine (T(4)) on the arterioles of hamster cheek pouch microcirculation in vivo. Microvessels were visualized using a fluorescent microscopy technique. Topical application of T(3) (3.08, 30.8, 61.5, 307, 615, and 6,150 nM/l) consistently induced dose-dependent dilation of arterioles within 2.0 +/- 0.5 min of administration. The application of T(4) (150, 257, 514, and 5,140 nM/l) caused different dose-dependent effects: dilation at the three lower doses within 16 +/- 2 min and rhythmic diameter changes at the highest dose. Aging of hamsters did not alter the arteriolar responses to T(3) and T(4). T(3)-induced dilation was countered by the inhibition of nitric oxide synthase with N(G)-nitro-L-arginine-methyl ester or N(G)-nitro-L-arginine. Iopanoic acid (IPA), which inhibits types I and II 5'-deiodinase, abolished the dilation elicited by 514 nM T(4) but did not affect T(3)-dependent dilation. 6-Propyl-2-thiouracil (PTU), which inhibits type I 5'-deiodinase only, did not affect the dilation induced by T(4). IPA and PTU did not impair arteriolar dilation induced by acetylcholine or sodium nitroprusside. These results indicate that T(3) induces arteriolar dilation, likely through nitric oxide release. The local conversion of T(4) to T(3) appears to be crucial for the dilation induced by T(4).

Thyroxine hair content in congenital hypothyroidism and hyperthyroidism

Using the determination of thyroxine (T4) hair content, we studied 16 hypothyroid newborns diagnosed by means of our regional screening program, and five hypothyroid infants, undetected at birth, at diagnosis and after 3 months of substitutive therapy (8-10 microg/kg/day L-thyroxine in newborns; 15 microg/kg/day in infants), and 13 hyperthyroid adults. Hair T4 content was similar at diagnosis in hypothyroid newborns (2.6 +/- 2.3 pg/mg hair) and in infants undetected at birth (2.4 +/- 1.7 microg/mg hair), but very high only in the latter after therapy (23.2 +/- 3.9 microg/mg hair). Untreated hyperthyroid adults surprisingly evidenced lower hair T4 (0.4 +/- 0.2 microg/mg hair) than controls (1.5 +/- 0.3 microg/mg hair). We suggest these findings are due to differential tissue storage of thyroid hormone, related to the different blood T4 concentration. Therefore, T4 hair assay could be a non-invasive method to further assess thyroid status.

Determination of thyroxine in the hair of newborns by radioimmunoassay with high-performance liquid chromatographic confirmation

Hair analysis is often used in forensic toxicology to study, retrospectively, chronic exposure of individuals to drugs, and consequently newborn hair may become an ideal sample to study intrauterine exposure to xenobiotics as well as to endogenous compounds. As a tool to investigate a supposed maternal thyroxine (T4) supply to the congenital hypothyroid fetus, we devised to use the analysis of T4 extracted from newborn hair. In the present paper, the analytical method based on T4 extraction from hair followed by a radioimmunoassay is described. To verify the nature of the T4-like immunoreactive material present in newborn hair, it was further studied by HPLC fractionation with radioimmunoassay of the eluted fractions. On the basis of a clear correspondence between retention times of T4 standard and T4-immunoreactive compound extracted from hair, we assigned this immunoreactive material to T4. Then, we determined T4 hair concentrations in 19 control newborns at birth and 12 congenital hypothyroid infants at 22 days of life. Values obtained from hypothyroid infants (31.47+/-8.8 pg/mg(hair), mean+/-S.D.) were not significantly lower than those obtained from healthy newborns at birth (36.10+/-13.2 pg/mg(hair)). Such results are in agreement with the hypothesis of a maternal supply of thyroxine to the fetus through placental crossing.

Determination of thyroxine in urine by cathodic stripping square-wave voltammetry

A square-wave polarographic method for the determination of the amino acid thyroxine in urine, after isolation from the matrix on a C18 Sep-Pak cartridge and subsequent elution through a column packed with Amberlite IRA-400 anionic resin, was developed. The optimal working conditions for the application of the method were found to be pH 10, provided by a carbonate buffer, and a potential of -0.690 V during 10 min. The equilibration time was applied during 5 s, and potential scans were performed at a square-wave amplitude of 100 mV and a scan rate of 300 mV s-1, with a square-wave frequency of 100 Hz. The measuring-system response was linear over the thyroxine concentration range from 10 to 60 ng ml-1 and the detection limit achieved was 2.71 ng ml-1. The relative error and relative standard deviation obtained were 1.18 and 4.67%, respectively.

A simple method for the separation and quantitation of radiolabeled thyroid hormones in thyroxine clearance studies

A method was developed to facilitate the separation and quantitation of radiolabeled thyroxine in plasma for thyroxine clearance studies. Following intravenous injection of radioactive thyroxine, the radiolabeled thyroid hormones were isolated from plasma protein and polar metabolites by solid phase extraction on a C18 sorbent bed. The individual thyroid hormones were then separated by ion-pair reversed phase chromatography and sequentially eluted through a UV detector and radiochromatographic detector. The radioactivity of individual radiolabeled thyroid hormones was corrected for recovery of carrier as determined from UV absorbance. The recoveries of thyroxine and 3,5,3'-triiodothyronine (T3) were 96% and 101%, respectively.

Thyroidal and peripheral production of 3,5,3'-triiodothyronine in humans by multicompartmental analysis

Multicompartmental analysis of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) kinetics based on the plasma disappearance curves of the two tracer hormones (J. J. DiStefano III, M. Jang, T. K. Malone, and M. Broutman. Endocrinology 110: 198-213, 1982 and J. J. DiStefano III, T. K. Malone, and M. Jang. Endocrinology 111: 108-117, 1982) was extended to include additional experimental data, namely, the appearance curve in plasma of labeled T3 generated in vivo from precursor T4. Kinetic analysis of data obtained in 14 studies carried out in normal subjects by using a composite six-pool model made it possible to quantify the contributions of the thyroid (3.3 micrograms.day-1.m-2) and the periphery (12.7 micrograms.day-1.m-2) to T3 production. T4 monodeiodination occurred mainly in peripheral tissues rapidly exchanging with plasma (10.7 micrograms T3.day-1.m-2), whereas only 2.0 micrograms T3.day-1.m-2 arose in slowly exchanging tissues. In contrast, if plasma disappearance curves only were analyzed, a value of 10.9 micrograms T3.day-1.m-2 was calculated for peripheral conversion in slowly exchanging tissues; this underscores the need for additional data, such as the [125I]T3 plasma appearance curve for the partition of central and peripheral production of T3.

Determination of L-thyroxine in reference serum preparations as the o-phthalaldehyde-N-acetylcysteine derivative by reversed-phase liquid chromatography with electrochemical detection

A simple procedure for the assay of L-thyroxine in serum preparations with D-thyroxine as internal standard is described. The L-thyroxine is extracted with acetonitrile, fractionated on a reversed-phase silica cartridge and analysed by reversed-phase high-performance liquid chromatography of the o-phthalaldehyde-N-acetyl-L-cysteine derivative. This derivative is not fluorescent, but may be detected with suitable sensitivity and selectivity with an electrochemical detector.